This invention relates to the manufacture of gas discharge devices, especially A.C. (alternating current) multiple gas discharge display/memory devices which have an electrical memory and which are capable of producing a visual display or representation of data such as numerals, letters, radar displays, aircraft displays, binary words, educational displays, etc.
Multiple gas discharge display and/or memory panels of one particular type with which the present invention is concerned are characterized by an ionizable gaseous medium, usually a mixture of at least two gases at an appropriate gas pressure, in a thin gas chamber or space between a pair of opposed dielectric charge storage members which are backed by conductor (electrode) members, the conductor members backing each dielectric member typically being appropriately oriented so as to define a plurality of discrete gas discharge units or cells.
In some prior art panels the discharge cells are additionally defined by surrounding or confining physical structure such as apertures in perforated glass plates and the like so as to be physically isolated relative to other cells. In either case, with or without the confining physical structure, charges (electrons, ions) produced upon ionization of the elemental gas volume of a selected discharge cell, when proper alternating operating potentials are applied to selected conductors thereof, are collected upon the surfaces of the dielectric at specifically defined locations and constitute an electrical field opposing the electrical field which created them so as to terminate the discharge for the remainder of the half cycle and aid in the initiation of a discharge on a succeeding opposite half cycle of applied voltage, such charges as are stored constituting an electrical memory.
Thus, the dielectric layers prevent the passage of substantial conductive current from the conductor members to the gaseous medium and also serve as collecting surfaces for ionized gaseous medium charges (electrons, ions) during the alternate half cycles of the A.C. operating potentials, such charges collecting first on one elemental or discrete dielectric surface area and then on an opposing elemental or discrete dielectric surface area on alternate half cycles to constitute an electrical memory.
An example of a panel structure containing non-physically isolated or open discharge cells is disclosed in U.S. Pat. No. 3,499,167 (incorporated herein by reference) issued to Theodore C. Baker, et al.
An example of a panel containing physically isolated cells is disclosed in the article by D. L. Bitzer and H. G. Slottow entitled "The Plasma Display Panel - A Digitally Addressable Display With Inherent Memory", Proceeding of the Fall Joint Computer Conference, IEEE, San Francisco, Cal., November 1966, pages 541-547 and also in U.S. Pat. No. 3,559,190 (incorporated herein by reference).
In the construction of the panel, a continuous volume of ionizable gas is confined between a pair of dielectric surfaces backed by conductor arrays typically forming matrix elements. The two conductor arrays may be orthogonally related sets of parallel lines (but any other configuration of conductor arrays may be used). The two arrays define at their intersections a plurality of opposed pairs of charge storage areas on the surfaces of the dielectric bounding or confining the gas. Thus, for a conductor matrix having H rows and C columns the number of elemental or discrete areas will be twice the number of elemental discharge cells.
In addition, the panel may comprise a so-called monolithic structure in which the conductor arrays are created on a signle substrate and wherein two or more arrays are separated from each other and from the gaseous medium by at least one insulating member. In such a device the gas discharge takes place not between two opposing elemental areas on two different substrates, but between two contiguous or adjacent elemental areas on the same substrate; the gas being confined between the substrate and an outer retaining wall.
It is also feasible to have a gas discharge device wherein some of the conductive or electrode members are in direct contact with the gaseous medium and the remaining electrode members are appropriately insulated from such gas, i.e., at least one insulated electrode.
In the prior art there exists D.C. (direct current) devices where the electrodes consist of an anode and a cathode which are typically in direct contact with the ionizable gaseous medium. It is also possible to construct such D.C. devices utilizing a dielectric overcoat, i.e., the same structure and configuration as an A.C. gas discharge display/memory panel described hereinbefore.
A wide variety of such devices exist in the prior art. Examples of such devices are disclosed in U.S. Pat. Nos. 2,142,106; 3,260,880; 3,720,452; 3,725,713; 3,237,040, and 3,497,751, all of which are incorporated herein by reference.
The present invention is intended to relate to the manufacture of all types of A.C. and D.C. display panels.
In addition to the matrix configuration, the conductor arrays of the display device (D.C. or A.C.) may be shaped otherwise. Accordingly, while the preferred conductor arrangement is of the crossed grid type as discussed herein, it is likewise apparent that where a maximal variety of two dimensional display patterns is not necessary, as where specific standardized visual shapes (e.g., numerals, letters, words, etc.) are to be formed and image resolution is not critical, the conductors may be shaped accordingly (e.g., a segmented digit display).
The gas is selected to produce visible light and invisible radiation which may be used to stimulate a phosphor (if visual display is an objective) and a copious supply of charges (ions and electrons) during discharge.
In the prior art, a wide variety of gases and gas mixtures have been utilized as the gaseous medium in a number of different gas discharge devices. Typical of such gases include pure gases and mixture of CO; CO.sub.2 ; halogens; nitrogen; NH.sub.3 ; oxygen; water vapor; hydrogen; hydrocarbons; P.sub.2 O.sub.5 ; boron fluoride; acid fumes; TiCl.sub.4 ; air; H.sub.2 O.sub.2 ; vapors of sodium, mercury, thallium, cadmium, rubidium, and cesium; carbon disulfide; H.sub.2 S; deoxygenated air; phosphorus vapors; C.sub.2 H.sub.2 ; CH.sub.4 ; naphthalene vapor; anthracene; freon; ethyl alcohol; methylene bromide; heavy hydrogen; electron attaching gases; sulfur hexafluoride; tritium; radioactive gases; and the so-called rare or inert Group VIII gases.
To obtain uniform resolution over the entire display surface of a gas discharge panel, it is imperative that the space between opposing walls of the gas envelope be uniform and that the walls of the chamber be sealed to provide a gas filled container.
Several methods of sealing such panels with various spacers therebetween appear in the prior art. For example, epoxy has been used as a sealant but produces impurities in the gas mixture which decreases the life of the panel. In situ fabrication of gas panels with granular solder glass as a sealant and spacers has also been utilized but uniform deposition of the solder glass is difficult and considerable numbers of cells are obliterated. In order to resolve these problems a soft glass rod or granular sealant and a hard glass rod spacer as has been used, as described in U.S. Pat. No. 3,778,127 (incorporated herein by reference) wherein the upper plate of the gas panel settles upon the spacing rods during a bakeout operation thus establishing a predetermined and uniform spacing within the envelope. However, such glass spacers are rigid, tend to crack and are detrimentally conspicuous to the observer.
Another glass sealing composition especially suitable for sealing together the two glass substrates of a multiple gas discharge display panel so as to provide a hermetically sealed ionizable gas chamber is disclosed in U.S. Pat. No. 3,734,702 (incorporated herein by reference) comprising a lead borosilicate solder glass containing 18% by weight of aluminum titanate which inhibits crystalization thus developing a seal with even stress concentration.
Several methods have been used in the past to seal the periphery of the two glass substrates of a gaseous discharge panel as well as maintain uniform gap spacing therebetween for the gas chamber. In one such method a composition in the form of a paste comprising a vitreous or devitrifiable solder glass with nitrocellulose and amyl acetate as the solvent-binder system is used as described in U.S. Pat. Nos. 3,088,834 and 3,127,278. In another method of sealing such substrates a solder glass is mixed with a vehicle consisting of equal parts of liquid poly (alpha methyl) styrene and terpineol in a ratio of6 parts by weight of glass to one part of vehicle resulting in a paste having a viscosity of about 50,000 to 90,000 centipoises which is admirably suited for screen printing the sealing glass on the outer edges of the substrates since complete burn-out and curing of the composition results at about 572.degree.F. as set forth in U.S. Pat. No. 3,625,733. A method of maintaining uniform gap spacing with low melting solder glass rods is described in U.S. Pat. No. 3,499,167 but such spacers tend to crush when the pressure and temperature fluctuates. Still another method of resolving the gap problem is claimed in U.S. Pat. No. 3,778,127 in which a soft glass rod sealant and a hard glass rod spacer is utilized wherein the upper plate of the gas panel settles upon the spacing rods during the bakeout operation thus establishing a predetermined and uniform spacing within the envelope. Unfortunately, such glass spacers provide no center support and are quite rigid thus tending to crack when stresses develope especially at high altitudes.
With the ever increasing use of gaseous discharge display panels in military aircraft and space ships which operate at altitudes above 35,000 feet, it has become imperative to develope a spacer system in such panels which will rectify the problems of spacer cracking and panel deformation at such altitudes.